Interferometric switch

ABSTRACT

A compact monolithic interferometric switch such as a Mach-Zehnder switch is formed such that one of the waveguide paths between the input and output couplers contains a material having exhibits a resonant nonlinearity, whereby its refractive index changes when pump power propagates through it. Each of the waveguide paths has a different propagation constant whereby signal light is subjected to a different delay in each path when no pump power is propagating through the rare nonlinear path. An input signal applied to the input of the switch appears at a first output terminal when the pump power does not propagate through the nonlinear path, and it appears at a second output terminal when the pump power is applied to the nonlinear path. Switching occurs at relatively low levels of pump power.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 08/489,090, filed Jun. 9, 1995, now abandoned, thedisclosure of which is hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to optical power switching devices.Optical switches with switching speeds up to 1 gigahertz are requiredfor numerous applications including local area networks, sensor arraysand communications systems. Many forms of optical switching devices havebeen developed. Typical examples are multiple-quantum-well waveguideswitches, strained-layer superlattice directional couplers and opticalfiber switches. These devices are based on the nonlinear effect of thematerial that forms them. In the case of semiconductor devices, therequired critical power for a switch is less than 1 mW. Until recentlyoptical fiber switches had been fabricated from optical fibers havingsilica based cores. The optical power required for these optical fiberswitches is on the order of several kilowatts since the nonlinearcoefficient of silica is extremely small.

The publication, P. L. Chu et al. "Optical Switching in Twin-CoreErbium-Doped Fibers", Optics Letters, Feb. 15, 1992, Vol. 17, No. 4, pp.255-257 reports that it was demonstrated that erbium-doped fiber has anonlinear coefficient approximately 1 million times greater than that offused silica. However, the large increase in nonlinear index inerbium-doped fiber is accompanied by a large absorption loss and aslowing of the response time. The switch disclosed by Chu et al.consists of a 2.26 m long piece of twin core erbium-doped optical fiber.It is difficult to input light to and output light from a twin core or adouble core optical fiber. Moreover, a long length of the Chu et al.Erbium-doped fiber is required. Also, low cross-talk cannot be achievedsince the power difference in the two cores affects the couplingmechanism. This is explained by Caglioti et al. in "Limitations toall-optical switching using nonlinear couplers in the presence of linearand nonlinear absorption and saturation", Journal of the Optical Societyof America B, vol. 5, No. 2, Feb. 1988, pp. 472-482. The two-core fiberrequires large power inputs or long fiber lengths that need to beconfigured in such a way as to prevent environmentally-induced phaseshifts such as bend-induced phase shifts.

The publication, R. H. Pantell et al. "Analysis of Nonlinear OpticalSwitching in an Erbium-Doped Fiber", Journal of Lightwave Technology,Vol. 11, No. 9, September 1993, pp. 1416-1424 discusses switchconfigurations employing both a Mach-Zehnder configuration and atwo-mode fiber configuration, each configuration utilizing anErbium-doped core.

Pantell et al. describe an experiment in which a 3.4 m length oftwo-mode fiber was utilized. A phase shift of π required an absorbedpump power of 15.5 mW. The signal was launched to inject approximatelyequal powers in the LP01 and LP11 modes. This type of signal injectionis difficult to implement, and the device is unstable with respect toexternal vibrations and perturbations.

One or both fibers in the phase shift region of the Mach-Zehnder deviceof Pantell et al. is made of Erbium-doped fiber, the pump power beingcoupled into only one of them. Since it is stated at page 1417 that thepump power requirement of a two mode fiber (TMF) switch is generallylarger than for an equivalent Mach-Zehnder (MZ) switch by a factor of2-4, it follows that a Mach-Zehnder switch of this type would be about85 to 170 cm long provided that the power remained constant. In theabsence of pump power all of the signal power appears at output port 2.When sufficient pump power is applied to cause a phase difference ofπ/2, the signal switches to output port 3. Pantell et al. indicate thatthe fiber core is heated due to the generation of phonons by the pumppower in the fiber core and that the two mode fiber is advantageous overthe Mach-Zehnder interferometer since the two modes utilize the sameguiding region and therefore react similarly to environmental changes.

The Pantell two-mode device is very sensitive to the launch conditionand any perturbations along the length of the two mode fiber. Also, itis not directly compatible with single-mode operation.

In order to attain compactness and ease of handling, it would beadvantageous for nonlinear switches of the Mach-Zehnder type to beformed as a monolithic structure. For such devices to be practical,their length should not exceed about 15 cm.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an opticalswitch that overcomes the heretofore noted disadvantages of prior answitches. A further object is to provide a compact, low power, lowcross-talk nonlinear optical switch.

Briefly, the monolithic interferometric switch according to one aspectof the present invention comprises input coupler means for splitting aninput signal into N equal signal components, where N>1. Combining means,having at least first and second output terminals, is provided forcombining the N components. N optical waveguide paths connect the Nsignal components to the combining means. At least one of the waveguidepaths contains a material having a resonant nonlinearity, whereby itsrefractive index changes when pump power propagates through it. Theinput coupler means and the combining means are free from nonlinearmaterial. The input coupler means, the combining means and the opticalwaveguide paths are in thermal contact with a matrix glass body.

In one embodiment the switch consists of first and second optical fibersextending longitudinally through an elongated body of matrix glass. Thebody includes a phase shift region and two spaced coupler regions atopposite ends of the phase shift region. The diameter of the body andthe diameters of the fibers are smaller in the coupler regions than inthe phase shift region At least that portion of the first fiber that isin the phase shift region contains a material having a resonantnonlinearity, whereby the refractive index of the first fiber changeswhen pump power propagates through it. The fibers have differentpropagation constants in the phase shift region in the absence of pumppower propagating through the first fiber so that the first fibersubjects the light propagating therethrough to a delay that is differentfrom the delay experienced by light propagating through the secondfiber.

According to a further aspect of the invention, an interferometricswitch such as a Mach-Zehnder switch or other intefferometric switch,includes first and second optical couplers and first and second opticalpaths extending between these couplers. The switch further includesmeans defining a first input port connected to the first coupler and afirst output port connected to the second coupler. At least one of thepaths is a nonlinear path incorporating a material having a resonantnon-linearity. The refractive index of the material and hence theoptical path length of the nonlinear path changes in response to opticalpower applied through the path. As further discussed below, the term"optical path length" is a measure of the time required for a lightsignal at a given wavelength and in a given propagation mode to passthrough the path from one end to the other. The optical path length isrelated to the physical length of the path and to the parameters of thepath which determine the phase velocity of light on the path, such asthe refractive indices of the materials in the path and the physicalconfiguration of the path. In the absence of optical pumping power, theoptical path lengths are different from one another. The device has azero-power transfer function relating the proportion of light suppliedthrough the first input port which appears at the first output port towavelength. As used in this disclosure, the term "zero-power transferfunction" refers to the transfer function which prevails under low-powerconditions, i.e., where the properties of the nonlinear path aresubstantially uninfluenced by pumping power. The zero-power transferfunction resembles the transfer function of a conventional Mach-Zehnderintefferometric device; it includes at least one peak at a firstwavelength and at least one valley at a second wavelength, and typicallyincludes a series of alternating peaks and valleys. At each peak,substantially all of the light supplied through the input port appearsat the first output port, whereas at each valley substantially none ofthe light applied through the first input port appears at the firstoutput port. Most preferably, the peaks and valleys of the zero-powertransfer function for wavelengths in an operating range corresponding tothe range of signal wavelengths to be processed are relatively closelyspaced, i.e., separated by a difference Δλ less than about 100 nmwavelength, preferably less than about 10 nm wavelength and mostpreferably, about 6 nm or less, typically between about 1 nm and about 6nm.

This aspect of the present invention incorporates the realization that,when the zero-power transfer function incorporates relatively closelyspaced peaks and valleys, only a small change in the optical path lengthof the nonlinear path is required to switch the device between a firstcondition, in which substantially all of the light at a given signalwavelength supplied through the input port appears at the first outputport and a second condition in which substantially none of the light atsuch signal wavelength applied at the first input port appears at thefirst output port. The degree of change in the path length required toinduce switching may be several orders of magnitude less than the degreeof change in path length required to induce switching in a comparabledevice wherein the optical path lengths at zero power are equal to oneanother. Such a comparable device has a flat zero-power transferfunction. One way to conceptualize the difference is that where thezero-power transfer function includes closely spaced peaks and valleys,only a small difference in the spacings between the peaks and valleyswill suffice to shift a peak into or out of alignment with the signalwavelength being transmitted.

The device according to this aspect of the invention provides severalsignificant benefits. The amount of optical power which must be appliedto the device to induce switching can be substantially less than thatrequired in a comparable device with a flat zero-power transferfunction. This greatly reduces the heat generated within the deviceduring operation and reduces thermal effects. Also, the physical lengthof the nonlinear path required to permit switching at a given level ofoptical power may be substantially shorter in the device according tothis aspect of the present invention than in a comparable device usinginitially equal optical path lengths. This, in turn, makes it practicalto provide the requisite switching action in a monolithic device such asdiscussed above.

Preferably, the device includes a second output port connected to thesecond coupler. The second output port may have a transfer functionsubstantially inverse to the transfer function of the first output port,so that the peaks of the second-output transfer function correspond tothe valleys of the first-output transfer function and vice versa. Thedevice including two output ports may be used to switch light at a givenwavelength selectively between two branches of a network, or to selectparticular wavelengths for diversion along one branch of a network. Thedevice with only a single output port may be employed as a tunablewavelength-selective filter.

According to a further aspect of the present invention, the principlesdiscussed can be applied in multi-stage intefferometric devices. Adevice according to this aspect of the invention includes at least threeoptical couplers and a plurality of optical paths extending between thecouplers. The paths are arranged in groups so that the couplers areconnected to one another in series by the groups of paths. Each groupincludes at least two paths in parallel with one another. One of thecouplers constitutes an input coupler at an input end of the serieswhereas another coupler at the opposite end of the series constitutesthe output coupler. The device further includes means defining a signalinput port connected to the input coupler and a first output portconnected to the output coupler. At least one of the paths in the deviceis a nonlinear path as discussed above having optical path length whichchanges when optical power propagates through the path. The path lengthsof the parallel-connected paths in at least one of the groups of paths,and preferably in all of such groups of paths, are different from oneanother in the absence of optical pumping power applied through thedevice.

Here again the device has a zero-power transfer function including apeak at a first wavelength and a valley at a second wavelength, andthese peaks and valleys desirably are spaced at close intervals asdiscussed above. Most preferably, each of the various groups of pathsincludes at least one nonlinear path having the nonlinear material. Thenonlinear material included in the nonlinear paths may be responsive tooptical power applied as radiation in a pumping wavelength range remotefrom the signal band encompassing the first and second wavelengths. Thecouplers may be adapted to direct light in this pumping wavelength rangesubstantially along the nonlinear paths.

The nonlinear paths may be directly connected with one another at eachof the couplers so that the nonlinear paths form a continuous nonlinearpath array extending through several couplers. The switch may furtherinclude a pumping input port connected to one end of this nonlinear patharray. The couplers desirably are effective to couple light in thesignal wavelength band between paths at each coupler but substantiallyineffective to couple light in the pumping wavelength band between thepaths. Thus, pumping light in the pumping wavelength band may be appliedthrough the pumping input port and will propagate substantially only inthe nonlinear path array. In a particularly preferred arrangement, allof the paths in the device are constituted by optical fibers, and thenonlinear path array is constituted by a first continuous fiberextending through the various couplers. The first continuous fiber mayinclude a plurality of spaced apart nonlinear sections incorporating thenonlinear material and a plurality of linear sections free of thenonlinear material, the linear sections extending throughout the variouscouplers. The device may further include a second continuous fiberextending through several couplers, the second continuous fiberconstituting a second path array including paths in each of severalgroups directly connected with one another. Preferably, the device, whenformed using fibers, includes a monolithic glass body surrounding thefibers in a thermal contact therewith. Each of the couplers may be anoverclad tapered coupler including narrowed portions of the fibers andthe body, the narrow portions of the body serving as the overclading ofeach such coupler.

Devices according to this aspect of the present invention can combinethe efficient switching action discussed above with any of the varioustransfer-function shapes associated with multi-stage interferometricdevices. One particular useful device of this type provides a relativelynarrow peak in the zero-power transfer function of one port withrelatively broad valleys on either side of such peak. As optical poweris applied to the nonlinear paths, the location of the peak shifts. Sucha device can be used to select one wavelength from among several in awavelength-division multiplexing system using several relativelyclosely-spaced signal wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art Mach-Zehnder switch.

FIG. 2 is a plot of power output vs. wavelength for two types ofMach-Zehnder devices.

FIG. 3 is a plot of the power (PS) required for switching as a functionof wavelength separation between adjacent peaks and valleys of the curveof FIG. 2.

FIG. 4 is a cross-sectional view of a Mach-Zehnder switch formed inaccordance with the present invention.

FIG. 5 is a cross-sectional view taken along lines 5--5 of FIG. 4.

FIG. 6 is a graph illustrating loss vs. launch power for a Mach-Zehnderinterferometric switch formed in accordance with the present invention.

FIG. 7 shows a planar Mach-Zehnder interferometric switch.

FIG. 8 is a schematic view depicting a device in accordance with afurther embodiment of the invention.

FIG. 9 is a graph depicting the zero-power transfer function of thedevice illustrated in FIG. 8.

FIG. 10 is a graph similar to FIG. 8 but depicting the transfer functionin the presence of optical pumping power.

DETAILED DESCRIPTION

A Mach-Zehnder switch is schematically illustrated in FIG. 1. A first orinput-end coupler 11 and a second or output-end coupler 12 areconcatenated by waveguide paths 14 and 15. The device includes first andsecond input ports 1 and 2 connected to the input coupler 11 and alsoincludes first and second output ports 3 and 4 connected to the outputcoupler 12. The couplers are usually 3 db couplers, whereby the signalpower that is applied to input port 2, for example, is evenly dividedbetween the two outputs of coupler 11, and thus directed along bothpaths 14 and 15 to the second or output-end coupler 12. At theoutput-end coupler, the signals directed along both paths are combinedwith one another. Depending upon the relative phase of the signalsarriving at output end coupler, the signal will appear at the firstoutput port 3 or at the second output port 4. One or both of waveguidepaths 14 and 15 contains a material having a resonant nonlinearity,whereby a refractive index change is induced by absorption of lightwithin a predetermined wavelength band. The rare earth elements areparticularly suitable since they exhibit large nonlinear refractiveindices. The rare earth element erbium exhibits a very large nonlinearindex. The use of neodymium as the nonlinear material would increaseswitching speed, but more switching power would be required. There arealso other dopants with which a population inversion can be achieved inorder to provide a resonant nonlinearity. Examples include thetransition metals such as chromium and titanium.

The light absorbed by the nonlinear material can be a pump or gatingpulse having a wavelength different from that of the signal.Alternatively, the signal wavelength can be within that band ofwavelengths that induces an index change in the nonlinear material. Inthis case, separate signal and gating pulses can be applied to one orboth input ports, or a single signal pulse can be applied to one inputport (as in the case of a power limiter), its amplitude determiningwhether switching occurs, i.e. it determines the output port at whichthe output signal appears. In the present discussion it is assumed thatwaveguide path 14 is the nonlinear path. Pump power is shown as beingapplied to input port 1, and the signal is shown as being applied toinput port 2. If desired, both pump and signal power could be applied tothe same input port.

In the Pantell et al. switch, the characteristics of coupler 11 are suchthat essentially all of the pump power applied to input port 1 remainsuncoupled whereby it propagates only in waveguide path 14. In theabsence of pump power applied to first input port 1, the signal appearsat first output port 3. This is accomplished by appropriately fixing thephase shift between the two waveguide paths 14 and 15 at zero. Statedanother way, the optical path lengths of paths 14 and 15 are equal inthe absence of pumping power. The pump power causes a change inrefractive index in waveguide path 14. When the pump is turned on withenough power to induce a difference in optical path length between paths14 and 15 equal to a phase shift of π at the signal wavelength, thesignal fully switches from output port 3 to output port 4.

Output power is plotted in FIG. 2 as a function of wavelength for twodifferent single-stage Mach-Zehnder devices. Curve 21 is a plot of thezero-power transfer function relating the proportion of light signalpower supplied through first input port 1 which appears at first outputport 3 to the wavelength of the light signal. The device represented bycurve 21 has paths 14 and 15 with optical path lengths of paths whichare equal in the absence of pumping power. That is, in the devicerepresented by curve 21, the product of the physical length andpropagation constant for both paths are equal when the optical powerapplied through the paths is low or zero. Curve 22 represents thezero-power transfer function relating the proportion of optical signalpower applied at the first input port 1 appearing at first output port 3to the wavelength of the optical signal for a device in accordance withan embodiment of the present invention. In the device according to thepresent invention, the optical path lengths of the two paths 14 and 15in the phase shift region between couplers 11 and 12 are significantlydifferent in the absence of pumping power. The difference in opticalpath length can be provided by a difference in the physical length ofthe paths; by a difference in the propagation constants of thewaveguides constituting the two paths, or both. At each peak 26 oftransfer function 22, substantially all of the optical signal powerapplied through first input port 1 appears at the first output port 3,whereas at each valley 25 of the transfer function, substantially all ofthe optical signal power applied at port 1 appears at the second outputport 4, and substantially none appears at the first output port 3.

Whereas curve 22 includes a plurality of peaks within the wavelengthrange shown, curve 21 is representative of a broadbanded characteristic,whereby only its peak appears within the wavelength range covered byFIG. 2.

The model discussed below shows that the amount of power required tocause a signal to switch between the two output ports of a Mach-Zehnderdevice is a function of the wavelength separation between a peak 26 andan adjacent valley 25, for example, of the zero-power transfer functionrepresented by curve 22 of FIG. 2. Thus, the optical power required forswitching is a function of the difference between the optical pathlengths of waveguide paths 14 and 15 in the absence of pumping. Themodel is derived for the case where the paths have the same physicalpath length z and different propagation constants β. As the propagationconstant of a waveguide is directly related to the effective refractiveindex n of the waveguide, the model is stated in terms of the effectiverefractive index. In order to calculate the power requirements, themodel assumes that waveguide paths 14 and 15 in the phase shift regionof FIG. 1 have different effective indices. Although the model assumesthat the nonlinear material is silica, similar results would be obtainedif it were silica doped with a material that enhanced the nonlinearproperty of the waveguide path.

The normalized output power for the device of FIG. 1 (before theintroduction of the gating signal) is

    P=COS.sup.2 (πz(n.sub.2 -n.sub.1)/λ)             (1)

where n₂ and n₁ represent effective indices of propagation in path 14and path 15, respectively, and λ is the signal wavelength. The length zof waveguide paths 14 and 15 is chosen so that at zero pumping power aπ/2 phase change is introduced between the two wavelengths of interest,i.e., between the desired wavelength of the peak and the desiredwavelength of the valley. If, for example, it is assumed that a maximumis to occur at wavelength λ₁ (point 26 of FIG. 2) and a minimum is tooccur at wavelength λ₂ (point 25 of FIG. 2), z is given by

    z= 2(n.sub.2 -n.sub.1)(1/λ.sub.1 -1/λ.sub.2)!.sup.-1(2)

The index change δ needed to cause switching at λ₁ is the index changedue to applied optical power required to alter the transfer function sothat a valley, rather than a peak, is aligned with wavelength λ₁. Thatis, δ is the amount by which the effective refractive index of one pathmust change, under the influence of the applied optical power, toprovide a transfer function as illustrated by curve 27, with a slightlydifferent period or difference in wavelengths between adjacent peaks andvalleys, such that a valley falls at λ₁. For a system with givenphysical path lengths and given zero-power indices n₁ and n₂, δ is givenby:

    πδz/λ.sub.1 =π(n.sub.2 -n.sub.1)z/λ.sub.1 -π(n.sub.2 -n.sub.1)z/λ.sub.2                   (3a)

so that

    δ×λ.sub.1  (n.sub.2 -.sub.1)/λ.sub.1 -(n.sub.2 -n.sub.1)/λ.sub.2)!                                (3b)

It is known that the rate of change in index per unit optical power perunit area, denoted n₂ * (also referred to as "nonlinear index") forsilica based fibers is

    n.sub.2 *=3.2×10.sup.-16 cm.sup.2 /watt              (4)

For a single mode fiber with an effective area of 75 μm², equation 4becomes

    n.sub.2 **=4.3×10.sup.-10 /watt                      (5)

The required power (in watts) for switching in a device with a silicabased nonlinear fiber is approximately

    P.sub.s =1.5δ/(4.3/10.sup.10)                        (6)

FIG. 3 is a plot of the power (P_(s)) required for switching as afunction of the PP Band, which is the separation in nm between a peak 26and an adjacent valley 25 of the transfer function shown in FIG. 2,(i.e., λ₁ -λ₂). Where the zero-power transfer function is a simpleperiodic function of wavelength as shown in FIG. 2, the peak-to-valleyseparation is one-half the period of the transfer function. The powerrequired to switch the device of FIG. 1, using silica as the nonlinearmaterial, would be about 1000 kW if the fibers 14 and 15 had similarpropagation constants in the absence of pump power, and if the devicetherefore had a flat or infinite-period transfer function. The plotshows that the power requirement for nonlinear switching in the deviceusing silica as the nonlinear material is reduced by a factor of 100 (toabout 10 kW) as the propagation constants of fibers 14 and 15 becomesufficiently different that the difference in wavelength between valley25 and peak 26 in FIG. 2 approaches 1 nm. For an erbium-doped or otherrare-earth doped nonlinear material, the value of n₂ * varies with theamount of doping and with wavelength. However, for typical erbium-dopedmaterials operating at the wavelengths commonly used for opticalcommunications, the value of n₂ * is approximately 10⁶ times the valueof n₂ * given above for silica, i.e., about six orders of magnitudegreater. Therefore, the power required to switch a device including theerbium-doped nonlinear material is about 10⁶ times (6 orders ofmagnitude less) than the power required for a corresponding device usingsilica as the nonlinear material. For a device using the erbium-dopednonlinear material but having a flat or infinite-period zero-powertransfer function, approximately 1 watt of optical power is required forswitching. By contrast, in a device according to a preferred embodimentof the present invention, using erbium-doped nonlinear material andhaving a zero-power transfer function with (λ₁ -λ₂) of about 1 nm, therequired optical power for switching is about 100 times less, i.e.,about 10 milliwatts.

Similar results would be obtained for a device in which the lengths ofthe optical paths in the phase shift region are different; thisconfiguration is often employed in planar devices.

It is preferred that the nonlinear material in the path exist only inthe phase shift region rather than continue into and form part of thecouplers so that the coupling characteristic is not affected by pumppower. Another important advantage of this configuration is that itenables the use of relatively high loss doped fibers or waveguides toachieve nonlinearity, but since the doped fiber exists only between thecouplers, loss is minimized. If the nonlinear material extends throughthe couplers, then the pump power should be applied to the fiber or paththat does not contain nonlinear material, the pump power being coupledto the nonlinear fiber; this would minimize loss.

In conventional Mach-Zehnder switches of the type disclosed in theaforementioned Pantell et al. publication, optical waveguide paths 14and 15 are relatively long, and problems arise as a result of theheating of the nonlinear path 14 when pump power propagates through it.In accordance with preferred embodiments of the present invention theheating problem is alleviated by forming the device as a monolithicstructure whereby heat generated by the nonlinear arm of the phase-shiftregion is conducted to the remaining arm of the phase-shift region. Sucha monolithic Mach-Zehnder device can be in the form of an overclad fiberstructure or a planar circuit. However, the length of the Pantell et al.device is such that it is not suitable for such monolithic devices. Forsuch monolithic devices to be practical, their length should not exceedabout 15 cm.

As discussed above, preferred devices in accordance with the presentinvention provide nonlinear switching at significantly lower powerlevels (up to two orders of magnitude lower than with the conventionaldesign disclosed in the Pantell et al. publication). Since there is atradeoff between length of nonlinear fiber and switching power, thisfeature can be employed to render the phase-shift region sufficientlyshort that the entire device is easily fabricated as an overclad orplanar structure. That is, the device can be shortened to an acceptablelength, for fabrication as a monolithic device and the switching powercan be correspondingly maintained at a relatively low level.

A monolithic overclad interferometric switch in accordance with thepresent invention can be formed in accordance with the teachings of U.S.Pat. No. 5,295,205 which is incorporated herein by reference. Themonolithic structure of FIGS. 4 and 5 contains concatenated overcladcouplers 41 and 42 that are joined by a phase shifting region 44. Thedevice is formed by inserting optical fibers 46 and 47 into the bore 48of a tube of matrix glass 49. Each of the optical fibers has a coresurrounded by cladding of refractive index lower than that of the core.In the illustrated embodiment, fiber 46 is a single piece of fiber, andfiber 47 consists of sections 47a, 47b and 47c which are fused togetherprior to making the device. Section 47a, which is located in phase shiftregion 44, is doped with rare earth ions, while sections 47b and 47c donot contain rare earth ions. That portion of fiber 46 that is located inthe phase shift region is designated 46a. Elongated, thin sections ofthe fibers extending through the couplers 41 and 42 form the couplingelements of each coupler. The coupling elements are disposedside-by-side parallel to one another so that light propagating alongeach coupling element can transfer to the adjacent coupling element byevanescent wave action. The ends 51a and 51b of fiber 46 define oneinput port and one output port, whereas the ends 55a and 55b of theother fiber 47 define the other input and output ports. Thus, input port51a is directly connected to the phase shift region 46a of fiber 46 eventhough the input port and phase shift region lie on opposite sides ofinput end coupler 41. As used in this disclosure with reference to twoelements disposed on opposite sides of a coupler, the term "directlyconnected" means that light can pass from one element to the otherwithout undergoing transfer between parallel light-conducting elementsof the coupler. Two fiber portions connected end-to-end through onecoupling element of a coupler are directly connected to one another. Bycontrast, input port 51a is connected to the phase shift region 47a ofthe other fiber indirectly, through coupler 41; light can pass from port51a to phase shift region 47a only by transfer between the narrowed,parallel fiber sections in coupler 41. The first output port 51b at theopposite end of fiber 46 is also directly connected to phase shiftregion 46a, whereas ports 55a and 55b are directly connected to phaseshift region or path 47a and indirectly connected to phase shift regionor path 46a.

The difference in propagation constants Δβ between the two fibers in thephase shift region 44 in the absence of pumping or switching power mustbe sufficient to enable switching at low power levels as discussedabove. Any technique for obtaining different propagation constants canbe employed. For example, the diameter of the core of fiber 47a can besmaller than that of fiber 46a as shown in FIG. 5. The different densityof dots in the cores of fibers 46 and 47 illustrates that the core offiber 47a contains rare earth ions. Alternatively, the fiber cores couldhave different refractive indices, or the fiber claddings could havedifferent refractive indices or diameters. Any two or more of thesefeatures can be combined. Assuming the aforementioned maximum acceptablelength of 15 cm and pump or switching power of less than 1 mW, then Δβwould be equal to or greater than 0.003.

The refractive index of that portion of the matrix glass tube adjacentthe fibers is less than the lowest refractive index of either of thefiber claddings. The bore can be provided with funnels (not shown) ateach end to facilitate insertion of the fibers. The combination of tubeand fibers is referred to as a coupler preform.

That portion of the tube between points a and b is initially heated andcollapsed onto the fibers and is at least partially fused to them. Also,the fibers are caused to contact one another, whereby there is goodthermal conductivity between them. This can be accomplished byevacuating the tube bore, heating the tube near a first end 53 to causeit to collapse at the region of applied heat, and moving the preformrelative to the heat source to gradually extend the collapsed regiontoward end 54 until the desired length of collapsed tube is obtained.Thereafter, coupler 41 is formed near end 53 of the tube by heating aregion of the tube and moving those sections of the tube on oppositesides of the hot zone in opposite directions to stretch the heatedregion. The stretching operation is stopped after a predeterminedcoupling is achieved. While stretching the tube to form the firstcoupler, optical power can be coupled to an input optical fiber, and theoutput signals can be monitored to control process steps in the couplermanufacturing process.

For best performance, couplers 41 and 42 have substantially identicalcoupling characteristics over the signal wavelength band of interest.The second coupler 42 is therefore preferably formed near tube end 54 bysubjecting the appropriate region of the tube to stretching conditionsthat are identical to those used to form the coupler 41.

A Mach-Zehnder interferometric switch was constructed in accordance withthe embodiment shown in FIGS. 4 and 5. Tube 10 was comprised of silicadoped with 5 wt. % boron. Fiber 46 was a standard single-mode fiberhaving an outside diameter of 125 μm and a core diameter of 9 μm. Thefiber cladding was formed of silica, and the core was formed of silicadoped with a sufficient amount of germania to provide a core-clad Δ of0.35%. Fiber 47 consisted of a single piece of erbium-doped fiber havingan outside diameter of 125 μm and a core diameter of 4 μm. The fibercladding was formed of silica, and the core was formed of silica dopedwith 1000 ppm by weight erbium and a sufficient amount of germania toprovide a core-clad Δ of approximately 1.0%.

The tube was collapsed onto the fibers and stretched to form couplers 41and 42 in accordance with the above-described method. The couplers were3dB at 1550 nm. The overall length of the resultant device was 12.7 cm.The peak to valley wavelength separation (see FIG. 2) of theMach-Zehnder switch was 6 nm in the absence of pump power.

A laser diode operating at 1521 nm was connected to input port 2 by anattenuator. This single source functioned as the signal and alsoprovided the power for changing the index of the erbium-doped fiber.FIG. 6 shows the output of the device as a function input power. Curve61 represents the device excess loss. Curve 62 represents the insertionloss between input port 2 and output port 4, and curve 63 represents theinsertion loss between input port 2 and output port 3. Essentially allof the input appeared at output port 3 when the input power was low, theinput switching to output port 4 as power level increased; FIG. 6 showsthat switching occurred at an input power of less than one milliwatt.The specific example shows that the amount of power needed to cause asignal to switch between the two output ports 3 and 4 of FIG. 1 dependson the phase difference already existing between the two arms 14 and 15of the phase shift region before the pump or gating pulse is introduced.

FIG. 7 shows that embodiment in which the interferometric switch isformed as a planar device. All waveguide paths and couplers are formedin or on substrate 66. Input paths 71 and 72 are connected to phaseshift paths 69 and 70 by coupler 68. Paths 69 and 70 are connected tooutput paths 73 and 74 by coupler 67. Path 70 is longer than path 69,whereby a phase shift is introduced between the signal componentspropagating through paths 69 and 70. The phase shift can also be inducedby providing paths 69 and 70 with different refractive indices orwidths. Although either of the paths 69 and 70 can be doped with a rareearth element, the shading on path 69 indicates such doping in thatpath. As described above, the refractive index of the doped path changeswhen pump power is introduced into the appropriate input path. Thiscauses an input signal introduced at input path 71 or 72 to be switchedfrom output path 73 to output path 74, for example.

Mach-Zehnder devices become increasingly more sensitive to temperatureas the wavelength separation between the peaks of the power output vs.wavelength curve becomes smaller. However, suitable overclad devices ofthe type shown in FIG. 4 having a peak separation as small as 3.5 nmhave been made, and devices having a peak separation of about 1 nm arepossible. This is possible because the fibers in the phase shift regionof the overclad structure are buried in the matrix glass. Thus, heatgenerated in the nonlinear fiber can conduct to the other fiber.Similarly, planar Mach-Zehnders are stabilized with respect temperaturebecause heat can conduct from one path to the other through thesubstrate. Moreover, because the pumping power input required to induceswitching is low for devices in accordance with the preferredembodiments of the invention, heat generation within the nonlinear pathis also low.

As schematically illustrated in FIG. 8, a device according to a furtherembodiment of the invention may include a plurality of stages in series.The device includes a first or input-end coupler 100, a final oroutput-end coupler 102 and intermediate couplers 104, 106, and 108. Thecouplers are connected to one another in series by a plurality of groupsof paths 110, 112, 114, and 116. Each group includes two paths connectedin parallel with one another. Thus, the first group 110 includes alinear path 110a having optical properties substantially independent ofthe optical power coupled through the path, and a nonlinear path 110bincorporating a nonlinear material with power-dependent refractive indexas discussed above. The other groups include similar linear paths 112a,114a and 116a and nonlinear paths 112b, 114b and 116b. The devicefurther includes a signal input port 118 and pumping input port 120connected to the input-end coupler 100, and first and second outputports 122 and 124 connect to the output-end coupler 102. All of thenonlinear paths 110b, 112b, 114b and 116b are directly connected to oneanother through the intermediate couplers to form a continuous nonlinearpath array. The pumping input port 120 is directly connected tononlinear path 110b through the input coupler 100 and thus directlyconnected to the nonlinear path array. The linear paths 110a through116a are also directly connected to one another at the intermediatecouplers to form a continuous linear path array, which in turn isdirectly connected to the signal input port 118. Couplers 100, 104, 106and 108 are arranged to couple light in a signal wavelength band betweenthe path arrays, so that light in the signal wavelength band applied atsignal input port 118 will be directed along the two paths 110a and 110bof the first group by input coupler 100, recombined at each succeedingcoupler and again directed along one or both paths of the next groupuntil the light reaches the output coupler 102, where it is directed tothe first output port 122 or to the second output port 124.

The couplers are arranged to direct light in a pumping wavelength band,different from the signal wavelength band, substantially only along thenonlinear path array. The nonlinear materials should be sensitive topumping power in the pumping wavelengths band. Where the device includesan erbium-doped nonlinear material, pumping wavelengths on the order ofabout 0.98 or 1.48 micrometers can be employed. Signal wavelengthspreferably are in the ranges most used for optical fiber communications,such as about 1.5 to 1.6 micrometers, and typically about 1.52-1.57micrometers. These signal wavelengths can also serve as pumpingwavelengths for erbium-doped materials. Preferably, the couplers arearranged to provide essentially no coupling between the path arrays forlight in the pumping wavelength band, so that light in the pumpingwavelength band supplied through pumping input port 120 will propagateonly through the directly connected elements, i.e., only through thenonlinear paths 110b-116b. The couplers may be evanescent couplers suchas the overclad couplers discussed above with reference to FIG. 4,having juxtaposed parallel elements such as the narrowed fiber sections.Selective coupling of the light in the signal band but not in thepumping band can be achieved by providing the coupling elements withdifferent dispersion characteristics. That is, the coupling elements arearranged to have substantially equal propagation constants for light inthe signal wavelength band, but markedly different propagation constantsfor light in the pumping wavelength band.

The parallel paths in each group of paths have different optical pathlengths at low power levels, i.e., in the absence of pumping powerpropagating through the nonlinear paths. Thus, with respect to thesignal wavelengths, the device acts as a multi-stage lattice filter forlight in the signal wavelength band. As described, in the Synthesis ofCoherent Two-Port Lattice Form Optical Delay-Line Circuit, Jinguji etal., J. Lightwave Technology, Vol. 13 No. 1, January 1995, pp. 73-82,the transfer function of a filter including interferometric devicesconnected in series depends upon the characteristics of the pathsconstituting the device as well as the characteristics of the couplers.The transfer function of the device can be selected by selecting thecharacteristics of the paths and couplers.

One particularly useful arrangement has all paths within each group alsoequal to one another in physical length. At zero power the paths of eachgroup have different effective refractive indices and hence differentvalues of β. The differences in optical path length may increaseprogressively from group to group in the direction from the input end ofthe device to the output end. The coupling ratios of the couplers mayalso differ from one another. As used in this disclosure with referenceto a coupler, the term "coupling ratio" means the ratio between thelight power transferred from one path to another within the coupler tothe light power remaining in its original path as it passes through thecoupler. For example, in a coupler with a 90/10 coupling ratio 90% ofthe optical power arriving at the coupler along one path is coupled tothe opposite path in the coupler, whereas 10% remains in its originalpath. Coupling ratios can also be stated as coupling percentage--thepercentage of the total arriving light coupled over to the oppositepath. Thus, a coupler with a 90/10 coupling ratio has 90% coupling,whereas a coupler of 15/85 coupling ratio has 15% coupling. In oneexemplary device, input coupler 100 has a 5/95 coupling ratio;intermediate couplers 104, 106 and 108 have coupling ratios of 15/85,25/75 and 15/85, respectively, whereas the output coupler 102 has a 5/95coupling ratio.

Such a device can provide a zero power transfer function, relating theproportion of light supplied through signal input port 118 to theproportion of light exiting through second output port 124 as shown bycurve 150 (FIG. 9). The zero-power transfer function between signalinput port 118 and the first output port 122 is shown by a curve 152,also in FIG. 9. Zero-power transfer function 150 includes a narrow peak154 at a wavelength λ₁ which in this instance is about 1.53 micrometersand valleys on the opposite sides of this peak. The valleys of thetransfer function are relatively broad and flat. The peak to valleyseparation or Δλ in such a case can be taken as the difference inwavelength between a point on the peak where the proportion of powerreaching the output port in question is 0.8 times the maximumproportion, and the wavelength in the valley where the proportion ofpower reaching the port drops below 0.2 times its maximum value. Thatis, the peak to valley separation Δλ mentioned above and discussed abovewith reference to FIGS. 2 and 3 should be taken as indicated in FIG. 9.

Here again, when pumping light is supplied, the effective index ofrefraction of the nonlinear paths changes and the transfer functionsalso change. The transfer function between signal input port 118 andsecond output port 124 while pumping power is applied is indicated at150' in FIG. 10, whereas the transfer function between the signal inputport and the first output port under the same pumping conditions isshown at the 152'. As in the embodiments discussed above, switchingoccurs when the effect of pumping power is sufficient to shift a peak ofthe transfer function to a wavelength formerly occupied by a valley orvice versa. For example, in the absence of pumping power, light at about1.542 micrometer wavelength falls in the valley of transfer function 150and in the broad peak of transfer function 152 (FIG. 9). Thus, if lightin this wavelength range is supplied through signal port 118, it will bedirected out of the device through the first output port 122. However,upon application of pumping power, through pumping input port 120, thetransfer function is modified to the form shown in FIG. 10 and light atthe 1.542 micrometer wavelength is switched to the second output port124.

Devices according to this nature can be used to good advantage inwavelength division multiplexing applications. Thus, a composite signalincorporating signals at several wavelengths can be supplied through thefirst or signal input port of the device and the optical pumping powercan be controlled to align the peak in the transfer function of thesecond output port with one or another of these wavelengths selectively.This selectively routes one of the signals through the second outputport while leaving the others routed through the first output port. Thedevice can also be provided with an additional filter or filtersdownstream from the output port to remove light in the pumpingwavelength band. Lattice or multi-stage devices of the type discussedabove with reference to FIGS. 8-10 can also be fabricated as monolithicdevices. Preferably, the devices can be made using the overclad approachdiscussed above with reference to FIG. 4. The linear path array andsignal input port may be formed from one fiber, whereas the nonlinearpath array may be formed from another continuous fiber. Preferably, thecontinuous fiber includes the nonlinear material only in sections 111spaced apart from one another and connected to one another by linearfiber sections 113 devoid of the nonlinear material, so that thenonlinear and linear sections are arranged in alternating order. Thelinear fiber sections form the narrowed fiber sections of the couplers.The overcladdings of all of the couplers may be formed integrally with atube overlying and encompassing all of the fibers.

Preferably, the sensitivity or difference in optical path length perunit of pumping power within the nonlinear path of each group isdirectly proportional to the zero-power optical path length differenceprovided by such group. Thus, path 112b provides a greater variation inoptical path length per unit of pumping power than path 110b, and path114b provides still greater sensitivity. Where the nonlinear path arrayincludes alternating linear and nonlinear sections, the linear sectionsof the nonlinear path array may have the same properties as the linearpath array. Where the path arrays are formed from fibers, the linearsections 113 of the nonlinear path array may be formed from lengths ofthe same fiber used to make the linear path array, whereas the nonlinearsections 111 may be formed from lengths of a nonlinear fiberincorporating the nonlinear material. In the absence of pumping power,the nonlinear fiber has different effective index of refraction than thelinear fiber. The optical path length difference in each group at zeropower is proportional to the length of the nonlinear fiber sectionincluded in the nonlinear path of the group. The sensitivity of thenonlinear path in each group is also proportional to the length of thenonlinear fiber section. Accordingly, the sensitivity of the non-linearpath in each group will be directly proportional to the zero-poweroptical path length difference in such group.

Numerous variations and combinations of the features discussed above canbe utilized without departing from the present invention as defined bythe claims. Merely by way of example, a single-stage device, or each ofthe groups of fibers in a multi-stage device may include more than twopaths extending parallel to one another. In a three path device, forexample, one path in the phase shift region would be free from rareearth ions, the second path would have some rare earth ions, and thethird path would have twice the amount of rare earth ions as the secondpath. Each of the paths in the phase shift region would delay the signala different amount, the first path providing the least delay and thethird path providing the most delay. Here again, the sensitivity of eachpath can be made proportional to the zero-power phase shift (delay). Amethod of making a N path intefferometric device (N>2) is disclosed inU.S. Pat. No. 5,351,325. Also, in a multi-stage device the nonlinearmaterial may be provided in less than all of the groups of paths. Thatis, some of the path groups may include only linear paths, desirablywith different optical path lengths. However, to enhance the sensitivityof the device to optical pumping power, it is preferred to include thenonlinear paths in all of the path groups. Moreover, it is not essentialto provide the second output. That is, where the device is used only asa filter to block selected wavelengths, it need have only one output.The other, unused output, can be terminated with a standardantireflective termination. In each of the embodiments discussed above,each port is directly connected to one path. However, the input ports,output ports or both can be indirectly connected to the paths throughthe couplers. For example, a separate input fiber defining an input portcan be indirectly connected to both paths 110a and 110b (FIG. 8) by theinput-end coupler 100. As these and other variations and combinations ofthe features discussed above can be utilized, the foregoing descriptionof the preferred embodiment should be taken by way of illustrationrather than by way of limitation of the invention as defined by theclaims.

What is claimed is:
 1. A monolithic Mach-Zehnder switch comprisinginputcoupler means for splitting an input signal into N equal signalcomponents, where N>1, combining means for combining said N components,said combining means having at least first and second output terminals,N optical waveguide paths connecting said N signal components to saidcombining means, at least one of said waveguide paths containing amaterial having a resonant nonlinearity, whereby the refractive index ofthe path changes when pump power propagates through it, said inputcoupler means and said combining means being free from said material,and a matrix glass body, said input coupler means, said combining meansand said optical waveguide paths being in thermal contact with saidbody.
 2. A monolithic Mach-Zehnder switch in accordance with claim 1wherein there is a difference Δβ between the propagation constants ofsaid waveguide paths such that each of said N waveguide paths subjectsthe light propagating therethrough to a delay that is different from thedelay experienced by light propagating through each of the otherwaveguide paths when no pump power is propagating through said at leastone waveguide path.
 3. A monolithic Mach-Zehnder switch in accordancewith claim 2 wherein Δβ is equal to or greater than 0.003.
 4. Amonolithic Mach-Zehnder switch in accordance with claim 1 wherein thelength of said matrix glass body is no greater than 15 cm.
 5. Amonolithic Mach-Zehnder switch in accordance with claim 1 wherein saidpaths are optical fibers, and wherein said fibers, said input couplermeans and said combining means are surrounded by an elongated body ofsaid matrix glass.
 6. A monolithic Mach-Zehnder switch in accordancewith claim 5 wherein said input coupler means and said combining meansare regions in said body wherein the diameter of said body and thediameters of said fibers are smaller than the diameters thereof in saidphase shift region.
 7. A monolithic Mach-Zehnder switch in accordancewith claim 1 wherein said matrix glass body comprises a planarsubstrate, said paths, said input coupler means and said combining meansbeing located at the surface of said substrate.
 8. A monolithicMach-Zehnder switch in accordance with claim 1 wherein said materialhaving a resonant nonlinearity is a rare earth.
 9. A monolithicMach-Zehnder switch comprisingan elongated body of matrix glass, firstand second optical fibers extending longitudinally through said body, aphase shift region in said body, two spaced coupler regions in said bodyat opposite ends of said phase shift region, the diameter of said bodyand the diameters of said fibers being smaller in said coupler regionsthan in said phase shift region, at least that portion of said firstfiber that is in said phase shift region containing a material having aresonant nonlinearity, whereby the refractive index of said first fiberchanges when pump power propagates through it, said fibers havingdifferent propagation constants in said phase shift region in theabsence of pump power propagating through said first fiber so that saidfirst fiber subjects the light propagating therethrough to a delay thatis different from the delay experienced by light propagating throughsaid second fiber.
 10. A monolithic Mach-Zehnder switch in accordancewith claim 9 wherein said material having a resonant nonlinearity is arare earth.
 11. An interferometric switch comprising first and secondoptical couplers and first and second optical paths extending betweensaid couplers, and means defining a first input port connected to saidfirst coupler and a first output port connected to said second coupler,said paths having optical path lengths L1 and L2, respectively, betweensaid couplers, at least one of said paths being a nonlinear pathincluding a material having a resonant nonlinearity such that therefractive index of said material and the optical path length of thenonlinear path changes when optical pumping power propagates throughsuch path, said path lengths L1 and L2 being different from one anotherin the absence of said optical pumping power.
 12. A switch as claimed inclaim 11 having a zero-power transfer function relating the proportionof light supplied through said first input port which appears at saidfirst output port to wavelength in the absence of said optical pumpingpower, said transfer function including a peak at a first wavelength atwhich substantially all of the light appears at said first output portand a valley at a second wavelength at which substantially none of thelight appears at said first output port said first and secondwavelengths have a difference Δλ therebetween of about 10 nm or less.13. A switch as claimed in claim 12 wherein said first and secondwavelengths have a difference Δλ therebetween about 6 nm or less.
 14. Aswitch as claimed in claim 13 wherein Δλ is between about 1 nm and about6 nm.
 15. A switch as claimed in claim 11 further comprising a secondoutput port connected to said second coupler.
 16. A device as claimed inclaim 11 wherein said first and second optical paths are constituted byfirst and second fibers, said first and second fibers extending throughsaid couplers.
 17. A device as claimed in claim 16 wherein portions ofsaid fibers extend beyond said couplers and constitute said ports.
 18. Adevice as claimed in claim 11 wherein said paths and couplers are formedas a monolithic unit.
 19. An interferometric switch comprising at leastthree optical couplers and a plurality of optical paths extendingbetween said couplers in groups so that said couplers are connected toone another in series by said groups of fibers, each such groupincluding at least two said paths in parallel with one another, one ofsaid couplers being an input coupler at one end of said series, anotherone of said couplers at an opposite end of said series being an outputcoupler, the device further including means defining a signal input portconnected to said input coupler and a first output port connected tosaid output coupler, each said path having an optical path lengthbetween said couplers, at least one of said paths being a nonlinear pathincluding a material having a resonant nonlinearity such that therefractive index of said material and the path length of the nonlinearpath changes when optical power propagates through such path, said pathlengths of the paths in at least one said group being different from oneanother in the absence of said optical pumping power, said device havinga zero-power transfer function relating the proportion of light suppliedthrough said first input port which appears at said first output port towavelength in the absence of said optical pumping power, said zero-powertransfer function including a peak at a first wavelength at whichsubstantially all of the light appears at said first output port and avalley at a second wavelength at which substantially none of the lightappears at said first output port.
 20. A switch as claimed in claim 19wherein each said group includes at least one nonlinear path having saidnonlinear material.
 21. A switch as claimed in claim 20 wherein saidcouplers are adapted to direct light in a signal wavelengths bandincluding said first and second wavelength through all of said paths andto direct light in a pumping wavelength band outside said signalwavelength band only along said nonlinear paths, said nonlinear materialbeing responsive to pumping radiation in said pumping wavelength band toalter its index of refraction.
 22. A switch as claimed in claim 21wherein said nonlinear paths are directly connected with one another ateach of a plurality of said couplers to form a continuous nonlinear patharray extending through a plurality of said couplers, the switch furthercomprising a pumping input port directly connected to said nonlinearpath array, said couplers being effective to couple light in said signalwavelength band between paths but substantially ineffective to couplelight at a pumping wavelength outside of said signal wavelength bandbetween paths, whereby pumping light at said pumping wavelength appliedto said pumping input port will propagate through only said nonlinearpath array.
 23. A switch as claimed in claim 22 wherein said paths areconstituted by optical fibers, said nonlinear path array beingconstituted by a first continuous fiber extending through said pluralcouplers.
 24. A switch as claimed in claim 23 wherein said firstcontinuous fiber includes a plurality of spaced-apart nonlinear sectionsincorporating said nonlinear material and a plurality of linear sectionsfree of said nonlinear material, said linear sections extending throughsaid couplers.
 25. A switch as claimed in claim 24 further comprising asecond continuous fiber extending through a plurality of said couplers,said second continuous fiber constituting a second path array includingpaths in each of a plurality of said groups.
 26. A switch as claimed inclaim 25 wherein all of said paths are constituted by said first andsecond continuous fibers.
 27. A switch as claimed in claim 23 furthercomprising a monolithic glass body surrounding said fibers, each saidcoupler being an overclad tapered coupler including narrowed portions ofsaid fibers and said body.
 28. A switch as claimed in claim 22 whereineach said group of paths includes a linear path having opticalpropagation properties substantially insensitive to pumping power, thelinear paths of each said group being directly connected to one anotherand constituting a linear path array.
 29. A switch as claimed in claim22 wherein said first and second wavelengths have a difference Δλtherebetween of about 10 nm or less.
 30. A switch as claimed in claim 22wherein said first and second wavelengths have a difference Δλtherebetween about 6 nm or less.
 31. A switch as claimed in claim 30wherein Δλ is between about 1 nm and about 6 nm.
 32. A switch as claimedin claim 28 further comprising a second output port connected to saidoutput coupler.
 33. A switch as claimed in claim 32 wherein each saidgroup consists of one said nonlinear path and one second path, each saidcoupler other than said output coupler being adapted to split lightarriving at such coupler substantially equally between the pathsextending to the next coupler, said output coupler being adapted tocouple light arriving at said coupler unequally to said first and secondoutput ports.